Dextromethorphan, also known by its chemical name (+)-3-methoxy-17-methyl-(9α,13α,14α)-morphinan, is currently one of the most widely used antitussives.
In addition to the physiological activity noted above, dextromethorphan is also an agonist of the σ2 receptor, an N-methyl-D-aspartate (NMDA) antagonist, and an α3β4 nicotinic receptor antagonist. Dextromethorphan inhibits neurotransmitters, such as glutamate, from activating receptors in the brain. Uptake of dopamine and serotonin are also inhibited.
Dextromethorphan is approved for use in over the counter cough suppressant products. It is currently in Phase I clinical trials for treating subjects with voice spasms, and Phase III clinical studies for treating Rett Syndrome (http://www.clinicaltrials.gov). Dextromethorphan is being studied with other drugs in a Phase II clinical trial characterizing pain processing mechanisms in subjects with irritable bowel syndrome (http://www.clinicaltrials.gov/). Dextromethorphan is also in Phase I clinical trials for treating hyperalgesia in methadone-maintained subjects (http://www.clinicaltrials.gov/).
In addition, a combination of dextromethorphan hydrobromide and quinidine sulfate is currently in Phase III clinical trials for treating diabetic neuropathic pain (http://www.clinicaltrials.gov). This drug combination, also know as Zenvia®, is in Phase III clinical trials for treating Involuntary Emotional Expression Disorder (IEED), also known as pseudobulbar affect, in subjects suffering from Alzheimer's disease, stroke, Parkinson's disease and traumatic brain injury (http://www.clinicaltrials.gov).
Dextromethorphan is metabolized in the liver. Degradation begins with O- and N-demethylation to form primary metabolites dextrorphan and 3-methoxy-morphinan, both of which are further N- and O-demethylated respectively to 3-hydroxy-morphinan. These three metabolites are believed to be therapeutically active. A major metabolic catalyst is the cytochrome P450 enzyme 2D6 (CYP2D6), which is responsible for the O-demethylation reactions of dextromethorphan and 3-methoxymorphinan. N-demethylation of dextromethorphan and dextrorphan are catalyzed by enzymes in the related CYP3A family. Conjugates of dextrorphan and 3-hydroxymorphinan can be detected in human plasma and urine within hours of its ingestion.
Dextromethorphan abuse has been linked to its active metabolite, dextrorphan. The PCP-like effects attributed to dextromethorphan are more reliably produced by dextrorphan and thus abuse potential in humans may be attributable to dextromethorphan metabolism to dextrorphan. (Miller, S C et al., Addict Biol, 2005, 10(4): 325-7., Nicholson, K L et al., Psychopharmacology (Berl), 1999 Sep. 1, 146(1): 49-59., Pender, E S et al., Pediatr Emerg Care, 1991, 7: 163-7). One study on the psychotropic effects of dextromethorphan found that people who are extensive metabolizers (EM's) reported a greater abuse potential compared to poor metabolizers (PM's) providing evidence that dextrorphan contributes to dextromethorphan abuse potential (Zawertailo L A, et al., J Clin Psychopharmacol, 1998 Aug., 18(4): 332-7).
A significant fraction of the population has a functional deficiency in the CYP2D6 enzyme. Thus, because the major metabolic pathway for dextromethorphan requires CYP2D6, the decreased activity results in much greater duration of action and greater drug effects in CYP2D6-deficient subjects. In addition to intrinsic functional deficiency, certain medications, such as antidepressants, are potent inhibitors of the CYP2D6 enzyme. With its slower metabolism in some people, dextromethorphan, especially in combination with other medication(s), can lead to serious adverse events.
A longer than recommended duration of a drug in the body may provide continued beneficial effects, but it may also create or prolong undesired side effects. Undesirable side effects at recommended doses of dextromethorphan therapy include nausea, loss of appetite, diarrhea, drowsiness, dizziness, and impotence.
Dimemorfan, an analog of dextromethorphan, also known by its chemical name as (+)-(9α,13α,14α)-3,17-dimethylmorphinan, is a non-narcotic antitussive. The antitussive activity of dimemorfan is believed to result from direct action on the cough center in the medulla (Ida, H., Clin Ther., 1997, March-April; 19(2): 215-31).
In addition to its antitussive properties, dimemorfan has been shown to have anticonvulsant and neuroprotective effects possibly arising from N-methyl-D-aspartate (NMDA) antagonism of dextromethorphan (DM) and/or high-affinity DM σ receptors (Chou, Y-C. et al., Brain Res., 1999, Mar. 13; 821(2): 516-9). Activation at the σ-1 receptor has been found to provide anticonvulsant action in rats and mice, like DM, but without the behaviorial side effects produced by DM and its metabolite, dextrorphan (Shin, E. J. et al., Br J Pharmacol., 2005, April; 144(7): 908-18 and Shin, E. J. et al., Behavioural Brain Research, 2004, 151: 267-276).
Metabolism of dimemorfan in humans is known to proceed through cytochrome P450 catalyzed N-demethylation as well as 3-methyl oxidation. Greater than 98% of a dose of dimemorfan is metabolized in healthy human males and none of the metabolites have been shown to have antitussive effects (Chou Y-C., et al., Life Sci., 2005, Jul. 1; 77(7): 735-45 and Chou Y-C., et al., J Pharm Sci., 2009, Jul. 1-15).
Additionally, two ether analogs of dextromethorphan, [(+)-3-ethoxy-17-methylmorphinan]also referred to herein as “dextroethorphan,” and [(+)-3-(2-propoxy)-17-methyl-morphinan]also referred to herein as “dextroisoproporphan,” have shown anticonvulsant activity (Newman, A. et al., J Med Chem., 1992, 35(22): 4135-42 and Tortella, F. et al., J Pharmacol and Exp Therap., 1994, 268(2): 727-733) as well as neuroprotective effects in rats (Tortella, F. et al., Neurosci. Lett., 1995, 198(2): 79-82).
Accordingly, it is desirable to provide new compounds that have the beneficial activities of dextromethorphan, dimemorfan, dextroethorphan and dextroisoproporphan and may also have other benefits, e.g., reduced adverse side effects, with a decreased metabolic liability to further extend its pharmacological effective life, enhance subject compliance and, potentially, to decrease population pharmacokinetic variability and/or decrease its potential for dangerous drug-drug interactions or decrease the likelihood of dextromethorphan abuse due to the formation of untoward metabolites such as dextrorphan.